Ferromagnetic thin-film memory element and a method of recording information therein

ABSTRACT

A ferromagnetic thin-film memory, made of two film layers of ferromagnetic material in which the thickness d, and anisotropy field constant Hk1 of the one layer and the thickness d2, anisotropy field constant Hk2 and saturation magnetization Ms2 of the second layer are selected so that the constant Hk2 is much larger than the constant Hk1 and satisfy the relationship: D1 X D2 X Ms2 X Hk2 1 X 10 6erg/cm 2 X 10 5 AND THE METHOD OF RECORDING ON THE FILM.

United States Patent [72] Inventor Eiichi Goto Tokyo-To, Japan [21] Appl. No. 872,470

[22] Filed Nov. 24, 1969 [45] Patented June 22, 1971 [73] Assignee Ziu'dan Hojin Parametron Kenkyusho,

Chiyoda-Ku v Tokyo-To, Japan [32] Priority Oct. 31, 1964 [33] Japan Continuation of application Ser. No.

505,278, Oct. 26, 1965.

[54] FERROMAGNETIC THIN-FILM MEMORY ELEMENT AND A METHOD OF RECORDING INFORMATION THEREIN 2 Claims, 12 Drawing Figs.

[52] U.S.CI 340/174 [51] lnt.Cl ..G11c11/14 [50] Field 01 Search 307/88; 340/174 [56] References Cited UNITED STATES PATENTS 3,193,694 7/1965 Ehresman et al. 307/88 3,193,806 7/1965 Pohm etal 340/174 Primary Examinen-Stanley M. Urynowicz, Jr. AttorneyRobert E. Burns ABSTRACT: A ferromagnetic thin-film memory, made of two film layers of ferromagnetic material in which the thickness d, and anisotropy field constant l-l of the one layer and the thickness 11,, anisotropy field constant l-i, and saturation magnetization M of the second layer are selected so that the constant H is much larger than the constant H, and satisfy the relationship:

diXfilX fl jYE Q I/E IE X and fli method of recording on the film.

PATENTEU JUN22 IQYE SHEET 1 BF 2 FIG.2

FIG.3

FIG. Hcn

FIG.4

FIG.5

PATENTED JUN22 197i SHEET 2 BF 2 FIG. 9(b) 9 .T 6 H 0 o a 4 I III] 0 m M u 9 m T 3 1| h 1 Ii: H F M M h/ 8 G e M M a 1 H Z 9 G F a" l 1| h: I 1 1 1 l 1|: M H n F m? l 2 X d d W H F rm 0 o X H fl 4 1 FIG.?

FIERMOMAGNE'I'IC THIN-FILM MEMORY ELEMENT AND A METHOD OF RECORDING INFORMATION THEREIN This application is a continuing application of U.S. Pat. application Ser. No. 505,278, filed on Oct. 26, 1965.

This invention relates to ferromagnetic thin-films and to memory devices wherein ferromagnetic thin-film memory elements are used. More specifically, the invention concerns a new ferromagnetic thin-film memory element of multiple layer construction and a method of recording information therein.

in general, a ferromagnetic thin-film as a memory element has unique features such as high switching speed and capability of affording nondestructive reading out. l-Ieretofore, however, thin films made of a single magnetic material have been used in almost all cases, and this practice has led to certain difficulties as will be described more fully hereinafter.

It is a general object of the present invention to overcome these difficulties and to achieve a marked improvement in exchange interaction therebetween whereby the memory characteristics of said memory element are improved.

According to the present invention there is further provided a method of storing information which comprises applying a rotating magnetic field within the film planes of a ferromagnetic thin-film memory element of the above stated character thereby to write in information.

The nature, principle, and details of the invention will be more clearly apparent from the following detailed description taken in conjunction with the accompanying drawings in which like parts are designated by like reference characters, and in which:

FIGS. 1a, 1b and 2 are graphical representations indicating characteristics for an explanation of the principle of memory storage in a known memory element;

FIGS. 3 and d are diagrammatic perspective views for an explanation of the principle of known memory storage methods;

FIG. 5 is a diagrammatic perspective view for a description of the principle of the present invention; and

FIGS. 6 through 10, inclusive, are graphical representations to be referred to for a description of the principle of the invention.

As conducive to a full understanding and appreciation of the nature and utility of the present invention, the following brief consideration of the memory characteristics of known thin films having uniaxial anisotropy is presented.

A magnetic thin film formed by electroplating or vacuum evaporation with an inclined incidence angle of an alloy of the pennalloy group (including those of approximately percent of iron, approximately 80 percent of nickel, and few percent of additives such as molybdenum and cobalt) in a magnetic field is easily magnetized in the direction of the so-called easy axis (hereinafter taken as the J's-axis direction within the film). The film is magnetized with difiiculty in thesocalled hard axis (hereinafter taken as the y-axis) direction which is perpendicular to the .r-axis direction.

In the ideal case wherein the magnetization of the film surface is uniform at all parts and rotates uniformly, the magnetic energy (unit: erg) per unit area is given by the following equatllOl'I.

where:

M, is the saturated magnetization (unit: gauss) of the film;

H is a material constant (unit: oersted) called the uniaxial anisotropy magnetic field;

H and H, are respectively x and y components (unit: oersted) of an external magnetic field;

6 is the angle formed between the magnetization vector and the x axis; and

d is the film thickness.

The magnetization characteristic in the x-axis direction in the case when H is applied in only the x direction as an external magnetic field is as shown in FIG. 1 (a). The magnetization characteristic in the y direction in the case when H is applied in only the y direction as an external magnetic field is as shown in FIG. I (b). In FIG. l, M and M, are x and y components, respectively, of magnetization.

The magnetization in the case when an external magnetic field is not being applied assumes either of two stable states, namely that wherein it is directed in the +x direction (which is caused to correspond to digit I and that where it is directed in the -x direction (which is causedv to correspond to digit 0). By these two states, single digits l and 0 of hinary number can be stored.

In order to change this stored state to write in new information, an external magnetic field H and H are simultaneously applied in the x-axis and y-axis directions and, moreover, in a manner such that they are outside of the asteroidal curve expressed by the following equation.

. HIZI3HUZIQ=Hk2IS This asteroidal curve is indicated by full line in FIG. 2.

In an actual case, however, the actual switching characteristic becomes such as that indicated by broken line in FIG. 2 because of shifting of the magnetic wall and dispersion of the anisotropic axis, and to write in l or 0" by the double coincidence of H, and H the values of the H and H, magnetic fields must be so adjusted as to lie within the relatively confined regions enclosed by the dotted lines. In a region l the numeral l is written in the film, and in the region 0, the numeral 0" is written.

While the reading out of information from a thin film of this character can be accomplished destructively, it is preferable to accomplish nondestructive reading out. One method therefor is to apply in only the y-axis direction a magnetic field H of suitable magnitude (the allowable maximum limit of which is I-I, in the case of an ideal asteroidal curve but must be held therebelow in an actual case to prevent erasure of information) to cause rotation of magnetization reversibly in the y direction, and to read out the variation of the x-component M I of the magnetization from the voltage induced in a coil wound in a manner to surround M When digit 1" is written in, M, decreases algebraically, whereas when digit 0 is written in, M, increases algebraically. Consequently, the waveforms of the voltages induced for 1" and 0" differ, whereby the information is read out on v the basis of this difference.

The above description relates to the principle of writing-in and reading out of information in a memory element of uniaxial isotropic character consisting of a single magnetic thin film. As stated above, such a memory device is disadvantageous in that the regions for writing in due to the double coincidence of H and H, are relatively confined.

Recording methods wherein, by using multiple magnetic thin films in place of the above described single magnetic film,

- the regions of writing in by double coincidence are expanded have been considered. Heretofore, however, only methods whereby the thin films are mutually coupled in a merely magnetic manner by demagnetizing fields have been considered.

The use of thin films in a manner to produce large demagnetizing fields is, in general, inconvenient for achieving stability of film magnetization and also because of the possibility of erroneous operation due to stray magnetic fields, but there is a mutually contradictory condition of the coupling also vanishing when the demagnetizing fields are erased. Accordingly,

"even when multiple layers of magnetic thin film are used, it is difficult to produce memory elements of stable operation by merely causing magnetic coupling.

According to the present invention, by the use ofa multiplelayer film formed by bonding together molecularly in intimate contact at least two magnetic thin films F, and F as shown in FIG. 5, there is provided a memory element having a wide writing-in region due to ,double coincidence and having, moreover, a stable operation. and, furthermore, there is provided a new information recording method depending on sequence of the applied fields.

For the sake of simplifying the following description, it will hereinafter be assumed that each film area is amply large relative to the film thickness, or that each film forms a closed magnetic circuit as in the cylinders shown in FIG. 4, thereby to reduce the size of the demagnetizing fields to a negligible order. Referring to FIG. 5, a coordinate axis 1 is taken perpendicular to the film planes, and axes x and y are taken parallel to the film planes. The two films F, and F, are assumed to have mutually different magnetic characteristics (since, if they had the same magnetic characteristics, they would become a single film because they are molecularly in intimate contact).

In the case when an external magnetic field H, is applied within the film planes, if there is absolutely no magnetic coupling between the two films F, and F,, the respective film magnetizations M and M will assume directions of respectively maximum stability at angles of 0, and 6, from the x axis. This effect is indicated by full line in FIG. 6, in which the ordinate represents distance in the direction of the film thickness, and the abscissa represents the directional angle (from the x axis) of magnetization.

. In general, the magnetization of a ferromagnetic material is imparted by the spin of electrons within the ferromagnetic material, and in a ferromagnetic material, neighboring spins tend to be parallely directed because of the ferromagnetic exchange interaction between the spins. In this embodiment of the invention, since two films are molecularly in intimate contact, the above stated effect causes the magnetization direction 0 to exhibit a continuous variation in the thickness z direction as indicated by the broken-line curve MT in FIG. 6 and a tendency to assume an average value intermediate between 6, and 0,. In FIG. 6, characters d, and d respectively denote the thickness of thin films F, and F,.

Thus, the ferromagnetic exchange interaction has a tendency to cause the magnetization directions of the molecularly contacting thin films to be parallel. The present invention is based on the practical utilization of this tendency. In a particularly important embodiment of the invention, a hard magnetic material having an anisotropy magnetic field H (either induced uniaxial anisotropy or crystal anisotropy), which is very much larger than the anisotropy field H of the film F, is used for film F,. Moreover, the thickness d, of this film is very much less than the thickness d, of the magnetically soft film F In this case, when an external magnetic field H,, which, while being much smaller than the field H is of an order sufficient to change greatly the magnetization direction of the softer film F, (there would be almost no change in the magnetization direction of the film F, if there were no ferromagnetic exchange interaction) is applied, the magnetization direction of the hard film F,, being attractively influenced by the magnetization of the film F also varies greatly.

This attractive influencing effect is remarkably pronounced when magnetic energy E, per unit area of the hard film F said energy E having the relationshipE,QdJ-I M, with M, being the saturated magnetization of the film F,, is of approximately the same magnitude as the increase E, in energy for the forcible twisting within the softer film, due to the exchange interaction, of the spins which have a tendency to become parallel. The energy increase E, is expressable by where A is the exchange contact and has a value of approximately l0 erg/cm. in the case of a metallic ferromagnetic material containing iron, nickel, or cobalt as a principal constituent.

The reason for this is that, when the energy E is excessively high, it becomes difi'icult to move the magnetization of the film F,,, and the aforesaid attractive influence does not occur. On the other hand, when the energy E is excessively small, the total magnetism of the double film is determined almost entirely by the magnetism of the soft film F Accordingly, the

following equation is obtained as the condition for effective utilization of attractive influencing effect.

d,d M,.,H K-IO erg/cm, (3) where: K is a constant determined experimentally;

d, and d, are in centimeters;

M, is in gausses; and

H is in oersteds.

A feature of the present invention is the utilization of the above described attractive influencing effect to change the shape of the asteroidal switching curve of the uniaxial anisotropic thin film indicated in FIG. 2 thereby to improve the memory characteristics of the memory element.

For a further exposition of the nature and principle of the present invention, the following description is set forth with respect to the case wherein both of the films F, and F have uniaxial anisotropy with easy (x) axes in the x-axis direction, and the materials thereof are so selected that the anisotropy magnetic field H of the film F is markedly larger than the anisotropy magnetic field H of the film F,.

If, under the condition defined by Equation 3), the value of K is suitable, it can be theoretically predicted that the switching characteristic will assume an asteroidal form which is elongated in the y-axis direction as shown in FIG. 7. The reason for this is that, in the case when, starting from the state wherein the magnetizations of films F, and F are both directed in the +x direction, an external magnetic field H is applied in the y-axis direction as indicated in FIG. 8, the average direction M, of the magnetization of the film F,, is directed toward the y-axis direction with greater difiiculty than that, M,, of the film F,, and, accordingly, even when a large external magnetic field H is applied, the magnetization can return reversibly to the original state without the loss of the stored information.

A double film of this character can be used by exactly the same method as that for an ordinary uniaxial anisotropic thin film, but in the case indicated in FIG. 7, in comparison with that indicated in FIG. 2, the allowable range of the magnetic field in the hard (y) axis direction in which writing in of information by double coincidence and nondestructive reading out is possible is increased. This feature was verified by the following experiment.

For the film F a permalloy (approximately 19 percent of iron and 81 percent of nickel) film was formed by electroplating in a magnetic field to a thickness d, of 1 micron, thereby an anisotropy magnetic field H,,, of approximately 3 oersteds was induced. On the films F, prepared according to the above process, films of approximately 40 percent nickel and 60 per- -cent cobalt and of respectively various thicknesses d were deposited as hard films F by electroplating in a magnetic field. Each film F had uniaxial anisotropy of H of approximately 30 oersteds. It was confirmed that, by the addition of the films F,, the switching characteristics were elongated in the y-axis direction, the maximum double-coincidence, magnetic field, allowable deviation (which was approximately twice that in the case of d =0) was obtained in the case of d =270 angstroms.

When the left-hand side of Equation (3) is computed from these quantities, the following result is obtained.

=l0"cm X2.7Xl0"cm X800 gauss X 30 oersted =65 Xl0erg/cm. Accordingly, K=6.5 is the optimum. However, the value of this d,, that is, the value of It, was found to be effective also when it was in the range of from one-fifth to 2 times the optimum value.

Furthermore, by the use of multiple-layer films of this character, depending on the conditions of their fonnation, a new writing method, that is, an infonnation recording method depending on the time sequence of magnetic fields applied within the plane of the films, whereby wiring in of information is accomplished for the first time when a rotating magnetic field is applied becomes possible. Whereas, reading out can be accomplished nondestructively by the same procedure described hereinbefore.

More specifically, when, in multiple-layer films F, and F of two soft and hard layers having substantially common easy (x) axis directions, the thickness d, of the hard film is increased beyond the stated condition just mentioned, it becomes impossible to influence attractively the magnetization of the film F to a sufficient degree unless the magnetization M, of the soft film F, is made rotate by applying external fields which form a cycle or a loop in the H,H, plane.

Let a magnetic field of waveforms as indicated in FIG. 9 be applied as H and H, where H,,,,.is the positive wave height of H, H,,, is the negative wave height of H, and H is the wave height of H, This magnetic field has a path or lows such that, in the H ,-H,, plane, it starts from the origin at a time t, as indicated in FIG. 10, becomes H in the H, direction at time t,, describes 2 cycle within the H,H,, plane, and returns to the origin at a time thus forming a cycle or a loopL.

When, on this path, the average directions M, and M of the magnetization vectors respectively of the films F, and F are both caused to start from the +x direction, they become as indicated in FIG. 10, and both finally become directed toward the -x direction. That is, the digit "0" is thereby written in.

The digit l can be written in by similar method by applying magnetic field such as to form a cycle with the right-hand half plane of FIG. 10, more specifically, by reversing the polarity of the magnetic field H,.

The existence of this new writing mode was verified by the following experiment. For the films F, and F films of an electrodeposited permalloy and a N,-C,, alloy permalloy under the same conditions as described above were used, and for a thickness d, of the soft permalloy film F, of 1 micron as before, a thickness d, of the hard film F of 810 angstroms was selected. For the magnetic field, pulses H H,,,, and H, (c.f. FIG. 9) each of l microsecond width were used.

Under these conditions, it was found that writing in (0" from 1'') occurs when the magnitudes of these pulses are within the following ranges:

the time period of pulse H, is shifted so that it coincides with either H or H,,,, whereby the magnetic field does not form a cycle. Since the thickness d in this case is 3 times that in the case of the aforedescribed computation (wherein d,=270 angstroms), the value ofK in Equation (3) is K=20. In general, a value of K of 5 or more was found necessary for this effect to appear.

A feature of a thin-film element of the above description is that information stored therein is not easily erased, and this element moreover, can be applied also to uses such as detection of the presence or absence specific waveforms written in by triple coincidence of the three pulses H l-{ and H As described above in detail, the present nvention affords a broadening of the region of double coincidence by providing multiple layers of magnetic thin films of magnetically different properties which films are molecularly placed in intimate contact by a method such as electrodeposition or vacuum evaporation. Moreover, it provides a memory element capable of accomplishing stable memory operation. Furthermore, the present invention provides, by the use of this memory element, anew method for writing information by using magnetic fields forming a cycle.

It should be understood, of course, that the foregoing disclosure relate to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What I claim is:

l. A method for writing information on a multiple layer memory element, said element having adjacent layers of unlike substances, disposed in intimate molecular contact with each other, comprising Rotating a magnetic field in an X,Y plane of said memory element, said rotating magnetic field being started at an origin in said X,Y plane, and being rotated through said X,Y plane in a cyclical manner to terminate at said origin, wherein said field is rotated in a cyclical manner by applying a first pulse to direct the field in one direction along the Y-axis, applying a second pulse to direct the field in one direction along the X-axis while maintaining said first pulse, terminating said first pulse while maintaining said second pulse, applying a third pulse to direct the field in the other direction along said Y-axis while maintaining said second pulse, terminating said second pulse while maintaining said third pulse, and then terminating sad third pulse to complete a cycle.

2. A method of writing on a multiple layer thin film memory element including abutting hard and soft layers disposed in intimate molecular contact and having substantially common easy-axis directions of magnetization along the X-axis of an X,Y plane, at the abutting faces of said layers, comprising applying external fields to rotate a magnetic field of the soft layer through a cycle by starting at an origin point in said X,Y plane and rotating the direction of magnetization to finally return to said origin point, wherein said magnetic field is rotated by applying a first pulse to direct the field in one direction along the Y-axis, applying a second pulse to direct the field in one direction along the X-axis while maintaining said second pulse, applying a third pulse to direct the field in the other direction along said Y-axis while maintaining said second pulse, terminating said second pulse while maintaining said third pulse, and then terminating said third pulse to complete a cycle. 

1. A method for writing information on a multiple layer memory element, said element having adjacent layers of unlike substances, disposed in intimate molecular contact with each other, comprising Rotating a magnetic field in an X,Y plane of said memory element, said rotating magnetic field being started at an origin in said X,Y plane, and being rotated through said X,Y plane in a cyclical manner to terminate at said origin, wherein said field is rotated in a cyclical manner by applying a first pulse to direct the field in one direction along the Y-axis, applying a second pulse to direct the field in one direction along the X-axis while maintaining said first pulse, terminating said first pulse while maintaining said second pulse, applying a third pulse to direct the field in the other direction along said Y-axis while maintaining said second pulse, terminating said second pulse while maintaining said third pulse, and then terminating sad third pulse to complete a cycle.
 2. A method of writing on a multiple layer thin film memory element including abutting hard and soft layers disposed in intimate molecular contact and having substantially common easy-axis directions of magnetization along the X-axis of an X,Y plane, at the abutting faces of said layers, comprising applying external fields to rotate a magnetic field of the soft layer through a cycle by starting at an origin point in said X, Y plane and rotating the direction of magnetization to finally return to said origin point, wherein said magnetic field is rotated by applying a first pulse to direct the field in one direction along the Y-axis, applying a second pulse to direct the field in one direction along the X-axis while maintaining said second pulse, applying a third pulse to direct the field in the other direction along said Y-axis while maintaining said second pulse, terminating said second pulse while maintaining said third pulse, and then terminating said third pulse to complete a cycle. 